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The hip implant is a device designed to duplicate the joint it is replacing, and to replicate as closely as possible the joint’s natural movement while standing up to stress without undue damage. Hip implants of one kind or another have been in use since the 1700s; modern hip arthroplasty was developed in the mid-20th century by British surgeon Sir John Charnley.
Requirements of All Hip Implants
All materials in an implant must be biocompatible (compatible with the human body) to minimize allergic or immune system reaction and avoid rejection of the device. Implants should also be resistant to corrosion, degradation, and wear, in order to retain their strength and shape and maintain functionality for a long time. Breakdown of any of the components leading to debris could cause damage not only to the device itself but also to the existing bone.
The key design features of each implant, including size, material, and dimensions, make each system unique. (For example, the size of the femoral head and acetabular component can vary depending on the durability of the material used. Smaller ball-and-socket sizes endure less friction but are more likely to dislocate; thus, advances in material durability have led to larger, more stable designs.) In addition, the same hip implant system will have different outcomes in different patients. Some implants are better suited to younger, active individuals, while others are more commonly used in older recipients. Factors that influence how long the implant lasts depend on the needs and characteristics of the individual as well as the type of implant chosen.
Metal and Ceramic Hip Materials
In the U.S., there are currently five types of devices available, classified based on the materials used for each surface.
The ball is made of metal and the socket is made of plastic (polyethylene) or has a plastic lining. These implants have been used for decades and consequently have the most long-term data. While a certain amount of wear and tear is inevitable with metal-on-polyethylene implants, that wear is relatively easy to track and advances since the 1980s have helped reduce it. In particular, the sterilization process (involving gamma radiation) can make polyethylene more prone to oxidation, which leads to decreased durability. Advances in packaging, sterilization and the polyethylene material itself have led to enhanced durability. Metal-on-polyethylene hip replacements are most often used by older, less active patients in their 70s and 80s, as well as younger patients whose active lifestyles may subject the implant to repetitive impact.
The ball is made of ceramic (generally alumina or zirconia) and the socket is made of plastic (polyethylene), or has a plastic lining. Alumina is highly wear resistant and consequently associated with decreased incidence of osteolysis, while zirconia is less durable due to the tendency of its crystalline structure to change over time. One disadvantage of ceramic implant materials is that they are more brittle than other materials and thus more prone to fracture. While alumina and zirconia have been used in implants for decades, oxidized zirconium material has recently been introduced, with promising early results and higher durability than traditional zirconia material.
The ball and socket are both made of metal. Metal-on-metal implants were some of the earliest modern hip implants, but fell out of favor due to poor materials and shoddy construction, which led to high failure rates. Advancements in material technology led to the metal-on-metal implants on the market today, which were intended to be more durable than other types of implants. However, the implants have been prone to side effects and failure, leading to a higher than expected rate of revision surgeries. Metal-on-metal implants have been subject to several voluntary manufacturer recalls, an outright ban in the United Kingdom, and tens of thousands of lawsuits against the five major manufacturers in the United States.
For more information, please see the Metal-on-Metal Hip Implants and Hip Replacement Recall pages. Information about specific metal-on-metal hip implants and their manufacturers, such as Stryker and DePuy, can be found linked in the Related Manufacturers sidebar to the left.
The ball is made of ceramic and the socket has a ceramic lining. Often used in younger, more active patients (though generally those who won’t subject the implant to repetitive impact), ceramic-on-ceramic implants experience significantly less wear than other types of implants. However, audible squeaking occurs in up to 2 percent (as high as 10 percent according to some studies) of patients, particularly younger, taller, more active recipients who place more stress on the joint. The implant requires more precise placement than other types; when revision surgery is required, it most often is to correct the device’s acetabular fixation. Ceramic-on-ceramic implants have been used more extensively in Europe than the United States.
The ball is made of ceramic and the socket has a metal lining. This type of implant received FDA approval in 2011 based on a study comparing 194 patients with DePuy’s ceramic-on-metal Pinnacle CoMplete Acetabular Hip System to a control group of 196 who received metal-on-metal implants. The study found no clinical difference between the two implants. Postmarket testing is ongoing.
- Hayashi, A. (2009, May). Bearing surfaces: Progress but not perfection. AAOS Now, Retrieved December 31, 2013
- U.S. Food and Drug Administration. (2011, June 14). FDA news release: FDA approves first ceramic-on-metal total hip replacement system. Retrieved December 31, 2013
- Sierra, R. J. (2009, February). Choosing the right total hip replacement. Mayo Clinic Health Letter: Online Edition, Retrieved January 6, 2014
- Wright, T. et al. (2009, November 10). Joint replacement: Implant bearing surface materials. Hospital for Special Surgery, Retrieved January 6, 2014
- Gomez, P. F., & Morcuende, J. A. (2005). Early attempts at hip arthroplasty: 1700s to 1950s. The Iowa Orthopaedic Journal, 25, 25-29. PMCID:PMC1888777